It is an object of the invention to provide an otoplastic which does not have the above-mentioned drawbacks.
In order to achieve this object the otoplastic in accordance with the invention is characterized in that the UV light-cured synthetic resin composition is provided with a ceramic filler comprising at least 50% by weight Al.sub.2 O.sub.3 (aluminum trioxide). Said otoplastic may be used in various types of hearing aids, such as behind-the-ear hearing aids and in-the-ear hearing aids. The otoplastic is a hollow envelope which, when used in a hearing aid, serves as a housing or a shell, dependent upon the type of hearing aid.
In experiments it has been found that an otoplastic in which the synthetic resin composition is provided with the desired quantity of said ceramic filler is very resistant to shocks caused by knocking or dropping the otoplastic, i.e., it is robust. In the course of the manufacture of the otoplastic, the Al.sub.2 O.sub.3 used is also responsible for the setting of the penetration depth of UV light. As a result, the addition of a colorant to the synthetic resin composition can be omitted. In order to counteract allergic reactions, preferably, the otoplastic in accordance with the invention does not contain colorants.
In an embodiment of the invention a UV light-cured acrylate composition is employed as the light cured synthetic resin composition. A UV light-curable acrylate somposition provided with a ceramic filler containing at least 50% by weight Al.sub.2 O.sub.3 has very suitable properties for use as the starting material for the manufacture of an otoplastic. Acrylates, to which a photoinitiator is added for the curing process, are colorless substances. After the addition of Al.sub.2 O.sub.3 and, subsequently, curing, an otoplastic having a chalk-white surface is obtained. Due to its clinical effect, such a surface has the advantage that soilage is rapidly detected, which has a favorable effect on the hygienic use of the otoplastic which, in turn, favorably influences the service life of the otoplastic and hence of the hearing aid in which the otoplastic is employed.
In a further embodiment of the invention the content of the ceramic filler ranges between 1 and 10% by weight of the synthetic resin composition. It has been found that a UV light-curable synthetic resin composition, in particular an acrylate composition to which 1 to 10% by weight of a ceramic filler including at least 50% by weight Al.sub.2 O.sub.3 are added, yields an otoplastic having a substantial mechanical strength. An optimum strength is obtained if the content of the ceramic filler is 5% by weight or substantially 5% by weight.
In a still further embodiment of the invention the ceramic filler comprises in addition to Al.sub.2 O.sub.3 at least on of SiO.sub.2, TiO.sub.2 and CaCO.sub.3. The degree to which SiO.sub.2, TiO.sub.2 and CaCO.sub.3 are used, is determined by the desired properties of the otoplastic. Although the percentages may be varied within wide limits, for example, to define the penetration depth of UV light during curing, the percentages by weight are customarily close to five.
The method in accordance with the invention is characterized in that before the UV light-curable synthetic resin composition is introduced into the mould, a ceramic filler including at least 50% by weight Al.sub.2 O.sub.3 is admixed with the synthetic resin composition. It should be noted that as used herein, the term UV light is to be understood to mean radiation originating from a radiation source having an emission maximum of (approximately) 360 mn. The otoplastic manufactured in accordance with the method of the invention has the above-mentioned properties and advantages. The method also has the advantage that it can be carried out using existing equipment which is known per se.
It is noted that EP-A 0498592 discloses a colored hearing-aid shell, which is manufactured from a visible light-curable synthetic resin composition including a filler of titanium oxide, silicon dioxide or potassium titanate, and a colorant. During curing said composition, visible light having a wavelength of 450 nm is used. Both the manufacturing method used and the resultant product differ, in essential features, from the method and the otoplastic in accordance with the invention.
Preferably, in the method in accordance with the invention, an UV light-curable acrylate composition is used as the synthetic resin composition. A suitable acrylate is, for example, a methacrylate or a urethane acrylate. Preferably, also a quantity of a ceramic filler in an amount of 1 to 10% by weight of the synthetic resin composition, in particular the UV light-curable acrylate composition, is added. To achieve optimum results, 5% by weight is preferred. At least one of the materials SiO.sub.2, TiO.sub.2, CaCO.sub.3 may be added to the ceramic filler in order to influence, in accordance with the requirements, specific properties of the composition and/or the otoplastic to be manufactured.
The invention further aims at providing a hearing aid which is provided with a sturdy otoplastic and which is comfortable to use.
To achieve this, the hearing aid in accordance with the invention is provided with the otoplastic according to the invention or is manufactured in accordance with the method of the invention.